Transmitter Mods
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Howard Fidel
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Howard,
Could you draw a little circuit diagram of your modifcation? Thanks -Jonathan KF6RTA
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Lawrence Galea
Hi Howard, Good job. The inductor reduces the feedback at higher frequencies while the capacitors offer a lower impedance at higher frequencies across R87 and R88. Can you try the same mod on the driver transistors Q92, Q93, Q96, Q97 Presently I don't have much time and I am very busy with other things. Regards Lawrence
On Tue, Apr 17, 2018 at 5:25 PM, Howard Fidel <sonic1@...> wrote:
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Jerry Gaffke
Very cool!
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Thanks for sharing this. Most of us have assumed it would take significantly more modding to bring the power up to something reasonable on the high bands. Jerry, KE7ER
On Tue, Apr 17, 2018 at 08:26 am, Howard Fidel wrote:
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Howard Fidel
Yes, I know. I just didn't get around
to it yet. Twice as many parts to solder.
On 4/17/2018 5:16 PM, Lawrence Galea wrote:
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MVS Sarma
inductor is likely to short the resistor if connected in parallel. I suppose that the concerned resistors have to be lifted and inductor to be in series to it. or have inductor inseries with a 0.1uF in series to connect that assembly parallel to the said resistors.Regards MVS Sarma
On Wed, Apr 18, 2018 at 2:46 AM, Lawrence Galea <9h1avlaw@...> wrote:
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Jerry Gaffke
The inductor is not in parallel with a resistor,
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Howard recommends placing it in series with a resistor:
On Tue, Apr 17, 2018 at 5:25 PM, Howard Fidel <sonic1@...> wrote: > 33 uH inductor in series with R86. I lifted the resistor mounting it on one pad standing up, > and teepeeing the inductor between the top of the resistor and the other pad. > Add a 270 pF capacitor across both R87 and R88. The two caps he added are in parallel with resistors. Jerry, KE7ER On Tue, Apr 17, 2018 at 09:16 pm, Mvs Sarma wrote:
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K9HZ <bill@...>
I read: “33 uH inductor in series with R86”
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From: BITX20@groups.io [mailto:BITX20@groups.io] On Behalf Of Mvs Sarma
inductor is likely to short the resistor if connected in parallel. I suppose that the concerned resistors have to be lifted and inductor to be in series to it. or have inductor inseries with a 0.1uF in series to connect that assembly parallel to the said resistors. regards sarma vu3zmv Regards
On Wed, Apr 18, 2018 at 2:46 AM, Lawrence Galea <9h1avlaw@...> wrote:
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Howard Fidel
If you read the instructions, it is
clear the inductor is in series with the resistor and provides
high frequency peaking. Howard
On 4/18/2018 12:16 AM, Mvs Sarma wrote:
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Thanks Howard for this simple mod. I did the mod this morning with spectacular results. Before the mod I had only 300mw at 13V on 28.500, after the mod I had 3.8 watts and after increasing the drive slightly I now have 4.5 watts. All of the other bands benefited from it also. All in all, a great mod. Thanks!
Joel N6ALT
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ohwenzelph
How and why does this mod work? What was the calculation that came up with 220pF and 27 uH?
For the inductor does it matter how many ma it can handle or what it’s self resonant frequency is? (doubt I will be able to follow the answer but still curious) thanks!
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Howard Fidel
The theory is very simple. The closed
loop gain of Q911 and Q912 is set by the ratio of R86/R85, or a
gain of 10. Adding an inductor into the path in the feedback loop
increases the effective impedance at higher frequencies, adding
"peaking" to the circuit, thus making the closed loop gain higher.
I think the poor frequency response is in the transformer as well
as the transistors hfe falling to under 25 typically at 30 MHz.
Also, the open loop gain is also increased by bypassing the
emitter resistors for higher frequencies. The transformer is a 2 to 1 step down, (it looks like a trifilar winding) so the theoretical gain of the stage is 5. Howard
On 4/19/2018 12:36 AM, ohwenzelph via Groups.Io wrote:
How and why does this mod work? What was the calculation that came up with 220pF and 27 uH?
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Gordon Gibby <ggibby@...>
howard -- thank you for that review! The inductor I find is up into the 2-3K (ohms reactance) range by 20 MHz so it definitely changes the feedback ratio at the higher frequencies. Capacitors across the emitter resistor likewise reduce the negative feedback
at higher F. This little improvement is one of the coolest things I've read yet about the uBitx power issues --- and also the idea of adding C across the power amplifier output transformer to better match ---
I'm encouraging our Local ARES group to move into these rigs as a learning platform and also for some, their first HF rig -- can't afford the pricey ones --- wonderful platform for LEARNING. Getting 10W or so on 80/40/30 would be great for NVIS emergency
communications for many of these people. At at a far far cheaper price than commercial fancy qrp rigs --- so next is to put one of mine in an ammo can for protection.
gordon
From: BITX20@groups.io <BITX20@groups.io> on behalf of Howard Fidel <sonic1@...>
Sent: Thursday, April 19, 2018 10:00 AM To: BITX20@groups.io Subject: Re: [BITX20] Transmitter Mods The theory is very simple. The closed loop gain of Q911 and Q912 is set by the ratio of R86/R85, or a gain of 10. Adding an inductor into the path in the feedback loop increases the effective impedance at higher frequencies, adding
"peaking" to the circuit, thus making the closed loop gain higher. I think the poor frequency response is in the transformer as well as the transistors hfe falling to under 25 typically at 30 MHz. Also, the open loop gain is also increased by bypassing the
emitter resistors for higher frequencies.
The transformer is a 2 to 1 step down, (it looks like a trifilar winding) so the theoretical gain of the stage is 5. Howard On 4/19/2018 12:36 AM, ohwenzelph via Groups.Io wrote: How and why does this mod work? What was the calculation that came up with 220pF and 27 uH?
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ohwenzelph
does it matter that most 27uH inductors seem to have a self resonant frequency below 28MHz?
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Gordon Gibby <ggibby@...>
I found some on digikey with a self resonaance 40 Mhz or higher....
probably does matter. At the self resonant point the series impeane will be very low....
From: BITX20@groups.io <BITX20@groups.io> on behalf of ohwenzelph via Groups.Io <Ohwenzelph@...>
Sent: Thursday, April 19, 2018 12:59 PM To: bitx20@groups.io Subject: Re: [BITX20] Transmitter Mods does it matter that most 27uH inductors seem to have a self resonant frequency below 28MHz?
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Howard Fidel
That is a good question. In our
application, the impedance is still high, it just becomes more
capacitive then inductive. If you look at the impedance plot, the
resonance is about 20 MHz, but the reactance at 30 MHz is still
2000 ohms, so the gain goes up. On the 15 meter band, it looks
like 30K ohms, so the amp runs open loop. The bottom line is that the design is not that robust to begin with, and what I have made is a patch. I would never have designed the signal path myself this way. I would have use an opamp to drive the transformer to replace everything from C80 to T10. The fix works, and has been validated by others. It would be smaller, probably not cheaper, but robust. Howard
On 4/19/2018 12:59 PM, ohwenzelph via Groups.Io wrote:
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Howard Fidel
Gordon: Very high, about 30 K. It actually helps us. Howard
On 4/19/2018 1:03 PM, Gordon Gibby wrote:
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Gordon Gibby <ggibby@...>
How is a series resonance 30,000 ohms?
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I’m kind of dumb, but I thought at series resonance , reactive components cancel and all that is left is the tiny ohmic resistance of the wire.
What am I missing?
Thanks
Gordon
On Apr 19, 2018, at 13:22, Howard Fidel <sonic1@...> wrote:
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Jerry Gaffke
Good question.
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It's a parallel resonance: https://www.everythingrf.com/community/what-is-self-resonant-frequency
On Thu, Apr 19, 2018 at 10:55 am, Gordon Gibby wrote: How is a series resonance 30,000 ohms?
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Gordon Gibby <ggibby@...>
Aha!!!
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Thanks. Didn’t realize it was parallel.
Much appreciated. Learn something every day!
Gordon
On Apr 19, 2018, at 14:00, Jerry Gaffke via Groups.Io <jgaffke@...> wrote:
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